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arms at the CP, so it was not surprising that both copolymers
had a narrow DT (2.0 and 1.2 8C), despite the broad PDI of
the polymers. The block sequence gave a somewhat greater
effect on the Dh with S-(O50D50)3-C6 giving a larger micelle
(130 nm) than S-(D50O50)3-C6 (95 nm) where the hydro-
philic blocks formed the micelle’s shell. When the hydropho-
bic DEGMA block is buried in the interior of the micelle it is
more poorly hydrated than when it is the shell layer of the
micelle, and so it is already collapsed or nearly collapsed, as
illustrated in Figure 10. However, the OEGA block is well
hydrated. Surprisingly though, the CPs of the two star
copolymers are quite different, with S-(D50O50)3-C6 having a
CP that at 57.5 8C is nearly 33 8C higher than the star copol-
ymer with the inverse block sequence, S-(O50D50)3-C6 which
DT supporting the concept of the hydrophobic block being
precollapsed and the small DT collapse of the hydrated
hydrophilic block. However, when the chain ends were mis-
matched to block polarity the results were contradictory.
The larger Dh and DT supported a less orderly micelle struc-
ture with a less orderly collapse, but the CPs were not con-
sistent. The mismatched triblock with hydrophilic chain ends
yielded a CP nearly identical to those of the triblock copoly-
mers with matched chain ends and blocks, while the triblock
with hydrophobic chain ends paired to hydrophilic blocks
gave a much higher CP, suggesting an efficient and orderly
hydration sphere.
Overall, the results are consistent with hydrophobic chain
ends controlling micelle formation and hydrophilic blocks
controlling hydration, and the data show that mismatching
the hydrophilicity of the block to that of the chain end leads
to disruption of the hydration sphere with a lowering of the
CP and a broadening of the temperature range for the chain
collapse. The architecture of a three-arm start reduced the
extent of these effects, but was still consistent overall. The
one exception to this was found for a symmetrical triblock
copolymer with hydrophobic chain ends paired to hydro-
philic blocks. The reason for this is not clear. Other analytical
methods such as Small Angle Neutron Scattering may be
needed to fully understand how pairing blocks with end
groups of different hydrophobicity affect the hydration,
aggregation and collapse of amphiphilic copolymers, but
given the likely growing importance of these EG-containing
monomers and their thermoresponse properties in the bio-
medical area, these materials are worthy of more study.
o
had a CP of 24.6 C. It is thought that neither the OEGA nor
the DEGMA blocks of the S-(O50D50)3-C6 copolymers are
well hydrated.
CONCLUSIONS
Di- and triblock copolymers were easily prepared by RAFT
polymerization using the hydrophilic OEGA and compara-
tively hydrophobic DEGMA with a series of different CTAs to
study amphiphilic copolymers with different architectures
and end groups. Linear diblock copolymers with amphiphilic
end groups allowed the thermoresponse properties of
diblock copolymers where the polarity of the blocks and
chain ends was matched, to be compared to those of diblock
copolymers with similar compositions but with block and
chain end polarity being mismatched. Other CTAs allowed
the synthesis of symmetrical triblock copolymers and
allowed the effect of block sequence to be studied in con-
junction with end groups that were matched or mismatched
with respect to block polarity, and star diblock copolymers
allowed the effect of block sequence with hydrophobic end
groups to be studied in an architecture that will facilitate an
orderly collapse of the chains.
ACKNOWLEDGMENTS
The authors express their sincere gratitude to Dr. Yadong Wang
at the Department of Bioengineering, University of Pittsburgh
for invaluable assistance obtaining the GPC data used in this
work.
When comparing the diblock copolymers the effect of pairing
end groups to blocks with different hydrophobicity is clear.
Pairing the hydrophilic block with the hydrophobic chain
end yields micelles with a lower CP and a higher Dh and DT.
The Dh is almost twice as large as when the hydrophobicity
of the block and chain end is better matched. When the
blocks and chain ends are matched the hydrophilic block
forms an efficient hydration sphere, giving the higher CP,
while the hydrophobic block is effectively collapsed giving a
smaller Dh and DT. However, mismatching disrupts the
hydration giving a lower CP, and the hydrophobic block with
hydrophilic chain end is also partially hydrated and perhaps
aggregated leading to a larger Dh and DT. The star polymer
shows a different trend because the architecture inhibits
aggregation, so the difference in Dh and DT is less significant
but placing the hydrophilic block on the outer shell clearly
allows a more effective and orderly hydration sphere as pro-
ven by the much higher CP. The triblock copolymers showed
slightly more complicated result. Matching chain ends to
block hydrophobicity/philicity yielded the smallest Dh and
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